For the electrophotographic process, the formation of the photoconducting recording element, which can also be called photo carrier or imaging unit, is particularly important. The basic principle of electrophotography can hereby be taken from U.S. Pat. No. 2,297,691, issued on Oct. 6, 1942, in the name of Chester F. Carlson, wherein a metal plate, on which a thin layer made of a photoconducting insulating material was deposited, has been used as a recording element.
Today, the recording element can include, for example, a drum that has a body made of aluminum, or a flexible strip, each of which has a suitable photoconducting sheath or coating. For the formation of the photoconducting layer (photoconductor)to occur, essentially three possibilities are considered. First, the layer can include arsenic triselenide (As2Se3) or similar materials containing selenium. Second, an organic photoconductor (organic photoconductor OPC) is considered. Third, amorphous silicon can be used for the photoconductor (a-Si, or also called α-Si). A coating with an organic multi-layer system is the most widespread method for producing a photoconductor.
For example, a photo carrier with an OPC coating can be used, that is homogenously negatively chargeable. For the imaging of this homogenously charged photo carrier, the photo carrier is then exposed in an appropriate manner. The light is absorbed in a charge-generating base coat. Positive charges are thus generated, that, by a charge transport layer, compensate for the negative charge on the surface in the image areas exposed at any one time. When a—Si or selenium coating is used, the surface becomes positively charged. In comparison to OPC layers, coatings made of a—Si have a higher wear resistance, but are burdened with higher production costs. OPC coatings can also be improved by using wear-reducing coatings.
Single-layer photoreceptors require the photo-generation and transport of electrons and holes in the same layer.
A vapor-deposited, semi-permeable metal carrier, made, for example, of Al, Ni, or Cr on a polymer carrier, such as, polyethylene terephthalate, is used as the substrate for electrophotographic strips. Metal cylinders serve as electrodes for imaging drums or cylinders. Suitable collars are generally pulled or stretched across them.
Normally, a thin blocking layer is inserted between the electrodes and the photoreceptor, in order to prevent a charge injection. This blocking layer should not be so thick that a residual charge accumulates during the charging/discharging cycles. To avoid hysteresis effects, blocking layers are normally less than 1 μm thick. The purpose of the blocking layer is to reduce the rate of dark discharge, to increase charge acceptance, and prevent point injections that could lead to local defects in the final image. Numerous insulating polymers have already been used as a blocking layer, including: acrylic polymers, epoxide resins, polyamides, polyester, polyphosphazene, polysiloxane, polyurethane, polyvinyls, etc. Bonding agents, namely, those with unsaturated bonding for bonding metal and resin, can likewise be used.
A photoconductor, as described above, including a blocking layer that contains a resinous material, suffers from a relatively high residual charge, and therefore, from a relatively low photosensitivity. Toner particles consequently, tend to adhere to non-imaged areas that have no electrostatic latent image, so that defective images are created, namely, “fuzzy” images. Such a phenomenon is particularly observed at relatively low temperatures, and at a relatively low atmospheric humidity. For the elimination of such a phenomenon, an intermediate layer or a bottom layer that includes resinous material and has conducting particles or metal oxides has been recommended. Alternatively, a bottom layer can be formed, by applying an oxide film to the conducting substrate using anodic oxidation. This is frequently used in highly reliable photoconductors, since oxide films are at beneficially high temperatures and high atmospheric humidity. A conductive base can also be oxidized, and saturated in an electrolytic solution. An oxide film can subsequently be molded on the conductive base by etching, for example.
On the other hand, oxidized aluminum surfaces are not homogenous. They exhibit a typical, sponge-like, or craterlike microstructure. This surface structure in turn leads to charge injection and charge collapse, which cause image defects, particularly, if the thickness of the blocking layer (or “barrier layer”) is significantly less than 1 μm.
A portion of a laser light used for imaging that falls onto a photoconductor, reaches the aluminum oxide film without being absorbed by a charge-generating layer. The light partially penetrates the oxide layer. The penetrated light is reflected at the boundary between the aluminum base and the aluminum oxide layer. A portion of the light does not penetrate the aluminum oxide film. This portion is reflected at the boundary between the charge-generating layer and the aluminum oxide film. Both reflected light portions have the same wavelength and are coherent. As a result, these light portions interfere with each other, which can lead to interference rings, depending upon variations in layer thickness. These types of interference rings lead to an irregular print density.